The present invention relates to a ferroelectric liquid crystal display device; more particularly, the invention relates to a liquid crystal display device wherein an image sticking phenomenon of a liquid crystal display screen is eliminated by preventing the threshold voltage of the liquid crystal from changing, and also relates to a method for driving the same.
In recent years, research and development has been carried out vigorously on liquid crystal display devices that utilize ferroelectric liquid crystals having properties, such as fast response speed and memory effect, that distinguish them from conventional liquid crystals. Liquid crystal display elements utilizing ferroelectric liquid crystals are finding widespread use in such applications as display apparatuses and liquid crystal shutters.
FIG. 1 is a diagram showing the arrangement of polarizers when the ferroelectric liquid crystal is used as a liquid crystal display element. A liquid crystal cell 2 is placed between the polarizers 1a and 1b arranged in a crossed nicols configuration in such a manner that the long axis direction of liquid crystal molecules when the molecules are in a first stable state or in a second stable state is substantially parallel to either the polarization axis, a, of the polarizer 1a or the polarization axis, b, of the polarizer 1b. 
When voltage is applied across the thus arranged liquid crystal cell, its light transmittance varies with the applied voltage, describing a loop as plotted in the graph of FIG. 2.
The switching of the ferroelectric liquid crystal, that is, a transition from one stable state to the other stable state, occurs only when a voltage of such a value that the product of the width value and crest value of the applied voltage waveform is larger than the threshold value is applied to the ferroelectric liquid crystal molecules. As shown in FIG. 2, either the first stable state (non-transmission (black) display state) or the second stable state (transmission (white) display state) is selected depending on the polarity of the applied voltage.
Here, the voltage value at which the light transmittance begins to change when the applied voltage is increased is denoted by V1, and the voltage value at which the light transmittance reaches saturation is denoted by V2; on the other hand, when the applied voltage is decreased and a voltage of opposite polarity is applied, the voltage value at which the light transmittance begins to drop is denoted by V3, and the voltage value at and beyond which the light transmittance does not drop further is denoted by V4. AS shown in FIG. 2, the second stable state is selected when the value of the applied voltage is greater than the threshold value of the ferroelectric liquid crystal molecules. When a voltage of opposite polarity greater in magnitude than the threshold value of the ferroelectric liquid crystal molecules is applied, the first stable state is selected.
When the polarizers are arranged as shown in FIG. 1, a white display (transmission state) can be produced in the second stable state and a black display (non-transmission state) in the first stable state. The arrangement of the polarizers can be changed so that the black display (non-transmission state) is produced in the second stable state and the white display state (transmission state) in the first stable state. The description hereinafter given, however, assumes that the white display (transmission state) is produced in the second stable state and the black display (non-transmission state) in the first stable state.
As one method for driving a ferroelectric liquid crystal display device, a time division driving method has been known in the art. In the time division driving method, a plurality of scanning electrodes and signal electrodes are formed on the substrates, and the liquid crystal display device is driven by applying voltages to the respective electrodes. FIG. 3 shows a driving scheme used in the time division driving method for driving the ferroelectric liquid crystal. As shown in FIG. 3, writing to a pixel is done by applying a scanning voltage (a) to its associated scanning electrode and a signal voltage (b) to its associated signal electrode, thereby applying their combined voltage (c) to the pixel. In the figure, (d) shows the light transmittance of the liquid crystal display device. Usually, each selection period (Se) is preceded by a reset period (Rs). NSe denotes a non-selection period. In the case of FIG. 3, the pixel is forcibly reset to the black display state (the first stable state) in the reset period (Rs) irrespective of its display data.
As shown in the left-hand half of FIG. 3, when producing a black display, a voltage that, as the combined voltage (c), does not exceed the threshold value is applied during the selection period (Se), and the state achieved in the reset period (RS) is thus retained. On the other hand, as shown in the right-hand half of FIG. 3, when producing a white display, a voltage that, as the combined voltage (c), exceeds the threshold value for the second stable state, is applied during the selection period (Se).
In the conventional art, it is known to apply during the reset period, a voltage scheme consisting of a single-polarity pulse or a bipolar pulse or a succession of bipolar pulses.
Ferroelectric liquid crystals exhibit spontaneous polarization and hence have the memory effect by which the written display state is maintained. However, if this memory effect is too strong, a phenomenon known as an image sticking phenomenon, in which the previously written display state persists and interferes with the writing of a new display state, becomes evident.
To prevent such an image sticking phenomenon, it has traditionally been practiced to form an alignment film using a material that has the property of preventing ionic impurities from adhering in the vicinity of the alignment film. Furthermore, when not using the display for a long period of time, it has been practiced to put the entire display screen into a mixed state of microscopic white or black regions and hold it in this state, thereby canceling, in the liquid crystal display screen as a whole, the internal electric field produced by the spontaneous polarization within the cell.
It has generally been believed that such an image sticking phenomenon occurs after the screen has been held in the same display state for a long period of time. The inventor, however, has discovered that one of the causes of image sticking phenomenon is a change in the threshold voltage required to cause a transition from the first stable state to the second stable state, and that the image sticking phenomenon can occur in a short period of time. It has also been found that the reset period employed in the prior art is not sufficient to prevent the image sticking phenomenon.
In view of the above situation, it is an object of the present invention to resolve the problem of image sticking phenomenon by providing a driving method which prevents such a threshold voltage change occurring in a short period of time.
To achieve the above object, the ferroelectric liquid crystal display element of the present invention comprises a ferroelectric liquid crystal between a pair of substrates, and a drive waveform for driving the liquid crystal includes a reset period having a switching period and a non-switching period which precede a selection period wherein, during the switching period, a voltage exceeding a threshold voltage required to switch the ferroelectric liquid crystal is applied irrespective of the display data to be written to a pixel, and during the non-switching period, an ion electric field in the ferroelectric liquid crystal is aligned in such a direction that it cancels the direction of an electric field produced by spontaneous polarization of liquid crystal molecules.
For example, a voltage applied during the non-switching period does not exceed the threshold voltage of the ferroelectric liquid crystal. Preferably, a voltage of 0 V, that is, no voltage, is applied.
The voltage applied during the switching period is a set of bipolar pulses.
The length xcex94T of the non-switching period is set so as to minimize the change in the threshold voltage.
The reset period is set with the same timing for all pixels.
Alternatively, the reset period is set each time display data is written to a pixel.
In that case, the length of the non-switching period is the same for all pixels.
At the end of a display operation of the ferroelectric liquid crystal display device, a switching pulse as applied during the reset period is applied to all the pixels to set them in the resulting state.
Using the ferroelectric liquid crystal display element of the invention or the driving method for the same, it becomes possible to suppress the change in the threshold voltage which occurs due to the display state of the liquid crystal panel. This serves to prevent the image sticking phenomenon caused by the change in the threshold voltage. This also serves to reduce the display flicker occurring during the non-selection period, allow a greater margin of driving with respect to temperature variations, and achieve a good display.